The proposed investigation aims to establish how sperm fertilizing capacity can be reversibly inhibited by membrane vesicles, which are present in the seminal plasma of various mammalian species including man, and by synthetic phospholipid vesicles containing cholesterol. Using the model system furnished by these phospholipid vesicles, it is intended to determine the physical and compositional properties that are necessary for their decapacitation effect on rabbit and rat sperm cells. Modified membrane vesicles may then be studied to clarify the nature of their influence on fertilizing capacity. It is planned in this connection to establish the effects of cholesterol elution and restoration, pronase digestion (to remove surface proteins), and prolonged sonication (to disrupt vesicle structure). Information concerning the interaction between the inhibitory vesicles and the sperm cell plasma membrane will be obtained. Vesicles bearing specifically labelled radioactive lipid (3H-galactose in glycolipid, 3H-cholesterol) and protein (125I-tyrosine in surface protein) shall be employed in this work. It is expected that it will be possible to demonstrate if vesicle fusion with the sperm cell occurs from the comparative amounts of protein and lipid bound to the cell. Partial characterization of protein and lipid in the sperm plasma membrane could indicate significant changes associated with decapacitation. Changes in plasma membrane fluidity with decapacitation will be appraised from the intensity of methylene and methyl peaks, attributable to fatty acid side chains in phospholipid, that can be seen in proton magnetic resonance spectra. These studies should advance understanding of the physiological control of fertilization.